Introduction
Glioblastoma (GBM) is the most aggressive and lethal primary brain tumor [
33]. Current standard-of-care, including surgery, radiotherapy, and chemotherapy, offers minimal clinical benefits for GBM patients with median survival of less than 16 months [
49]. The basis of therapeutic failure is the significant inter- and intra-tumoral heterogeneity of GBM [
34,
38,
48,
55,
56]. One aspect of heterogeneity is reflected by the transcriptional subtypes. GBMs have been stratified by bulk gene expression profiles into at least three subtypes, namely proneural, classical, and mesenchymal subtypes [
55,
56]. Among these subtypes, the mesenchymal subtype is associated with the worst prognosis and the presence of tumor-associated macrophages/microglia [
39,
56].
Another aspect of heterogeneity is reflected by the developmental state of GBM cells in the tumor. In this context, GBM stem-like cells (GSCs) present at the apex of cellular hierarchies and give rise to differentiated GBM cells (DGCs) [
1,
23]. GSCs possess capacities for self-renewal, differentiation, and tumor propagation in vivo and exhibit preferential resistance to radiotherapy and chemotherapies [
1,
13,
23]. GSCs are essential to drive tumor progression, but the importance of DGCs had been dismissed until a recent study showed that DGCs also contribute to tumor progression in collaboration with GSCs [
57]. This study highlighted the potential importance of DGCs in GBM propagation.
The GBM microenvironment consists of heterogeneous cells, namely tumor cells, including GSCs and DGCs, and non-tumor cells including endothelial cells, vascular pericytes, tumor-associated macrophages, and other immune cells [
11,
16,
22,
46]. Macrophages are an abundant cellular component of the GBM microenvironment and play multiple roles in GBM progression [
15,
22,
46]. Tumor-associated macrophages release several factors, including interleukin (IL)-6 and IL-10, which promote tumor cell growth, facilitate angiogenesis, and suppress the anti-tumor functions of other immune cells [
24,
45]. Additionally, GSCs and tumor-associated macrophages interact with each other closely [
46]. Tumor-associated macrophages secrete cytokines, such as pleiotrophin and TGF-β1, to maintain the stemness of GSCs and promote invasion of GSCs [
24,
46,
47]. GSCs recruit monocyte-derived macrophages from peripheral blood through paracrine periostin and osteopontin signaling [
46,
58,
63]. GSCs also promote the survival of M2 tumor-supportive macrophages by secretion of WISP1, which play immune suppressive roles in the tumor microenvironment, [
52]. The crosstalk between GSCs and macrophages has been explored actively, but the biological roles of DGCs in GBM progression, especially in the tumor microenvironment, are largely unknown.
Here, using DGC-specific transcriptomic signatures, we investigated the biological roles of DGCs in the tumor microenvironment, and demonstrate that DGCs accelerate GSCs-dependent tumor progression by shaping a mesenchymal microenvironment via CCN1-mediated macrophage infiltration.
Materials and methods
Unsupervised hierarchical clustering of TCGA GBMs (HG-UG133A) with the DGC signature was performed by Morpheus (
https://software.broadinstitute.org/morpheus). Single sample gene-set enrichment analysis (ssGSEA) scores were calculated using the single sample Gene Set Enrichment Analysis Projection (ssGSEAProjection) module in GenePattern (
https://cloud.genepattern.org) [
2]. Gene set enrichment analysis (GSEA) was performed using the GSEA desktop application (
http://software.broadinstitute.org/gsea/downloads.jsp) [
50]. To identify specific immune cells linked to DGC signatures or CCN1 in GBM, we examined TCGA GBM (HG-UG133A) dataset for 20 types of immune cells using validated gene set signatures [
7,
20]. Gene ontology enrichment analysis (GOEA) was performed through the GlioVis portal. To identify DGC-specific enhancer regions, deposited H3K27ac ChIP-sequencing data from three matched pairs (MGG4, 6, and 8) of GSCs and DGCs (GSE54047) [
51] were downloaded from the NCBI Gene Expression Omnibus (GEO) database. DGC-specific enhancer regions were defined by selecting all enhancers present in DGCs, but absent in GSCs using BEDTools [
42]. The H3K27ac signals in a 10 kb region of each site were visualized as heatmaps and metaplots using the plotHeatmap and plotProfile functions of deepTools [
43]. For
de novo and known motif enrichment analysis of DGC-specific enhancers, we used the HOMER software package [
26]. H3K27ac ChIP-sequencing enrichment plots at the CCN1 locus of three matched GSC and DGC pairs (GSE54047) were visualized using Integrative Genomics Viewer [
44].
Cell culture
The human GBM cell lines were U87ΔEGFR and U251MG provided by Dr. Balveen Kaur (University of Texas Health Science Center, Houston, TX). U87MG cells were purchased from the American Type Culture Collection. A172 and LNZ308 cells were provided by Dr. E. Antonio Chiocca (Brigham and Women’s Hospital, Boston, MA). Patient-derived GBM primary cultures MGG4, MGG8, MGG18, and MGG23 were provided by Dr. Hiroaki Wakimoto (Massachusetts General Hospital, Boston, MA). Normal human astrocytes (NHAs) were purchased from Lonza. U937 monocyte-like cells were purchased from the Japanese Cancer Research Resources Bank. Human embryonic kidney (HEK) 293FT cells were purchased from Thermo Fisher Scientific.
All DGCs, and U251MG, U87MG, U87ΔEGFR, A172, LNZ308, and HEK 293FT cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 100 U/ml penicillin, and 100 µg/ml streptomycin. All GSCs were cultured in neurobasal medium (Gibco) supplemented with 0.5 × N2 supplement, 1 × B27 supplement minus vitamin A (Gibco), 0.5 × penicillin/streptomycin/amphotericin B suspension (FUJIFILM Wako), 3 mM L-glutamine (Gibco), 2 μg/ml heparin (Sigma-Aldrich), 20 ng/mL human EGF (PeproTech), and 20 ng/mL human FGF basic (PeproTech). U937 monocyte-like cells were cultured in RPMI 1640 medium supplemented with 10% FBS, 100 U/ml penicillin, and 100 µg/ml streptomycin. NHAs were cultured in AGM BulletKit (Lonza) in accordance with the manufacturer’s instructions. All cells were maintained at 37 °C and 5% CO2, and confirmed to be free of mycoplasma. Cell lines were authenticated by Promega using short tandem repeat profiling in December 2016.
Isolation of GSCs and DGCs by fluorescence-activated cell sorting
MGG4 and MGG8 cells were washed with PBS, blocked with anti-CD16/32 antibodies and normal mouse serum in PBS for 30 min at 4 °C, and then labeled with a Brilliant Violet 421 anti-human CD133 Antibody (BioLegend, #372808). Then, the cells were incubated with propidium iodide, and CD133-positive and -negative cells were sorted by a BD FACSAria III Cell Sorter (Becton Dickinson). A Brilliant Violet 421 Mouse IgG1, κ Isotype Ctrl Antibody (BioLegend, #400158) was used as a negative control to determine the amount non-specific background staining. The sorted CD133-positive cells were cultured in the GSC medium described above. Matched CD133-negative cells were maintained in DMEM supplemented with 10% FBS to maintain their differentiation status. GSC phenotypes were validated by expression of stem cell marker SOX2, their self-renewal capacity (serial neurosphere passaging, in vitro limiting dilution assay), serum-induced cell differentiation, and tumor propagation capacity (in vivo limiting dilution).
DNA constructs and lentiviral transduction
Lentiviral vectors (LVs) expressing GFP (LV-GFP, Addgene, #26001) or RFP (LV-RFP, Addgene, #25999) were purchased from Addgene. For knockdown experiments, two non-overlapping shRNAs against human CCN1 and TAZ were cloned into pLKO.1 puro (Addgene, #8453). A non-targeting scramble shRNA (#1864, shCONT) and two non-overlapping shRNAs against human YAP (#42540 and #42541) were purchased from Addgene. The target sequences of shRNAs used in this study were as follows. shCCN1-1: CGAACCAGTCAGGTTTACTTA; shCCN1-2 [targeting 3ʹ-untranslated regions (UTRs)]: GGCAGCTATCTGCACTCTAAA; shTAZ-1: GCGATGAATCAGCCTCTGAAT; shTAZ-2: GCGTTCTTGTGACAGATTATA, shYAP-1: GCCACCAAGCTAGATAAAGAA; shYAP-2: CCCAGTTAAATGTTCACCAAT; shCONT: CCTAAGGTTAAGTCGCCCTCG. Lentiviral constructs overexpressing wildtype CCN1 (pTomo-CCN1) or TAZ (pTomo-TAZ) were generated by cloning human CCN1 or TAZ open reading frames, respectively, into the pTomo vector (Addgene, #26291). Site-directed mutagenesis was performed to produce lentiviral constructs expressing D125A, a CCN1 mutant defective for binding αvβ3/αvβ5 integrins, and DM, a CCN1 mutant defective for binding αMβ2/α6β1 integrins [
14,
32]. A control lentiviral construct was generated by cloning multiple cloning sites into the pTomo vector. For YAP overexpression, FUW-tetO-wtYAP (Addgene, #84009) and FUdeltaGW-rtTA (Addgene, #19780) were used. For the negative control, FUW-tetO-EGFP (Addgene, #84041) and FUdeltaGW-rtTA were used. HEK 293FT cells were used to generate lentiviral particles by cotransfection of packaging vectors psPAX2 (Addgene, #12260) and pMD2.G (Addgene, #12259) with TransIT-LT1 (Mirus Bio). For lentiviral transduction, cells were transduced with lentiviruses for 48 h and then processed for analyses.
Mouse intracranial tumor models
All animal experiments were performed with approval from the Committee on the Ethics of Animal Experimentation at Okayama University. For intracranial tumor xenograft models, female BALB/c-nu/nu mice (5–6 weeks old) were purchased from CLEA Japan Inc. The intracranial tumor xenograft models in mice were established as we described previously [
28]. Briefly, the mice were anesthetized and tumor cells were stereotactically injected into the right frontal lobe (3 mm lateral and 1 mm anterior from the bregma and 3 mm depth from the dura) using a stereotactic frame (Narishige) and Hamilton syringe (Hamilton). A mouse
de novo GBM model was generated by stereotactic injection of lentiviruses harboring H-Ras and shP53 [pTomo-HrasV12-IRES-GFP-shp53, the vector plasmid was a kind gift from Dr. Dinorah Friedmann-Morvinski (Tel Aviv University, Tel Aviv, Israel] into the hippocampus of transgenic mice expressing GFAP-Cre, FVB-Tg (GFAP-cre) 25Mes/J (The Jackson Laboratory, #004600) [
21]. Mice with neurological deficits or a moribund appearance including a hunched posture, gait changes, lethargy, and weigh loss were sacrificed. Following transcardial perfusion with 4% paraformaldehyde (PFA), brains were harvested, fixed in 4% PFA, and embedded in paraffin or cryopreserved in 30% sucrose for cryosectioning. To observe the effect of macrophage infiltration induced by CCN1, we used U87ΔEGFR, which has rapid tumorigenesis, in a preliminary experiment for the experiment using the GSC-DGC pair.
Human GBM tissue samples
Fresh GBM tumor tissue for qRT-PCR and immunofluorescence staining were obtained from primary GBM patients who underwent surgical resection at Okayama University Hospital. The study was approved by the ethical committee of the Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama, Japan (approval no. 1608-026). All patients included in the study had provided informed written consent.
Immunofluorescence staining
Tumor samples from GBM patients and mouse intracranial GBM models were fixed in 4% paraformaldehyde overnight at 4 °C, followed by overnight cryoprotection with 30% sucrose in PBS at 4 °C. Samples were then sectioned at a thickness of 7 µm. Sections were washed with PBS twice, permeabilized, and then blocked with 0.3% Triton X-100, 5% BSA in PBS for 1 h. Then, the sections were stained with primary antibodies against Iba1 (1 µg/mL, FUJIFILM Wako, #019-19741), CD206/MMR (2 µg/mL, R&D Systems, #AF2535), SOX2 (2 µg/mL, R&D Systems, #AF2018), and CCN1/Cyr61 (10 µg/mL, Novus, #NB100-356) overnight at 4 °C, followed by the secondary antibodies against rabbit or goat immunoglobulin G (IgG) labeled with Alexa Fluor dyes (Thermo Fisher Scientific) at room temperature for 1 h. After immunostaining, the samples were mounted with DAPI Fluoromount-G (SouthernBiotech, #0100-20). Images were obtained under an LSM780 confocal laser scanning microscope (Carl Zeiss).
TEAD luciferase reporter assay
For the TEAD luciferase reporter assay, 1 × 105 DGCs or GSCs were seeded in each well of a 24-well plate. After 12 h, the cells were transfected with the YAP/TAZ-responsive TEAD Firefly luciferase reporter vector 8 × GTIIC-luciferase (Addgene, #34615) (150 ng/cm2) and Renilla luciferase control reporter vector pGL4.74 [hRluc/TK] (Promega) (100 ng/cm2) using TransIT-LT1 (Mirus Bio) in accordance with the manufacturer’s instructions. At 24 h after transfection, Firefly and Renilla luciferase activities were quantified using the Dual-Luciferase Reporter Assay System (Promega) in accordance with the manufacturer’s instructions.
Enzyme-linked immunosorbent assay
Secreted CCN1 protein levels in conditioned media from paired GSCs and DGCs were quantified using an enzyme-linked immunosorbent assay (ELISA). DGCs or GSCs (1 × 106) were seeded in each well of a 12-well plate in DMEM supplemented with 10% FBS or GSC medium, respectively. After 12 h, the media were changed to fresh DMEM without FBS or fresh GSC medium and the cells were cultured for 24 h. At the end point, conditioned media were collected and analyzed using a Human Cyr61/CCN1 ELISA kit (RayBiotech, #ELH-CYR61) in accordance with the manufacturer’s instructions.
Conditioned medium preparation
Conditioned media (CM) were obtained by culturing GSCs or DGCs at 2 × 106 cells/mL in RPMI 1640 medium without serum for 24 h. The cells were removed by centrifugation at 2000 rpm at 4 °C for 10 min and the CM was sterile filtered through a 0.22-μm filter.
Migration assay
U937 cells were cultured in RPMI 1640 medium with 10% FBS for 24 h before priming. U937 cells were primed with 5 nM phorbol 12-myristate 13-acetate (Promega, #V1171) for 48 h to become monocyte-derived unpolarized macrophages. Migration assays were performed in 24-well plated with ThinCert cell culture inserts (8-μm pores, Greiner Bio-One) in accordance with the manufacturer’s instructions. Briefly, 5 × 105 primed U937 unpolarized macrophages suspended in serum-free culture medium were seeded in the upper chamber. Medium with recombinant human CCN1 protein (PeproTech, #120-25) or CM was added to the remaining receiver wells. Cells were then allowed to migrate for 48 h before fixation for staining with 0.05% crystal violet (FUJIFILM Wako, #031-04852).
Western blot analysis
Cells were collected and then lysed in cell lysis buffer (20 mM Tris–HCl, pH 7.5, 150 mM NaCl, 1 mM EDTA, 1 mM Na2EGTA, and 0.5% Triton X-100) containing a cOmplete Protease Inhibitor Cocktail (Sigma-Aldrich) and PhosSTOP phosphatase inhibitor cocktail (Sigma-Aldrich). After sonication, lysates were centrifuged at 15,000 rpm at 4 °C, for 10 min. The protein concentration of the supernatants was measured using a bicinchoninic acid protein assay (Thermo Fisher Scientific). The supernatants were added to a 1/3 volume of 4 × SDS sample buffer (240 mM Tris–HCl, pH 6.8, 8% SDS, 40% glycerol, 0.1% bromophenol blue, and 20% 2-mercaptoethanol) and boiled at 95 °C for 5 min. Equal amounts of protein samples were applied to SDS-PAGE and then transferred to a PVDF membrane (Immobilon-P, 0.45 μm) (MilliporeSigma). The membrane was blocked with 0.5% dry skim milk in TBST. After blocking, the membranes were incubated with primary antibodies overnight at 4 °C and then with secondary antibodies for 1 h at room temperature. The signals were developed with Clarity Western ECL Substrate (Bio-Rad Laboratories) and detected with a ChemiDoc imaging system (Bio-Rad Laboratories). The primary antibodies were anti-CCN1/CYR61(1:1000, Cell Signaling Technology, #14479), anti-YAP (1:1000, Santa Cruz Biotechnology, #sc-101199), anti-TAZ/WWTR1(1:1000, Sigma-Aldrich, #HPA007415), anti-SOX2 ((2 µg/mL, R&D Systems, #AF2018), and anti-GAPDH (1:1000, Sigma-Aldrich, #MAB374). The secondary antibodies were horseradish peroxidase-conjugated anti-mouse IgG (1:4,000, Cell Signaling Technology, #7076), anti-rabbit IgG (1:4,000, Cell Signaling Technology, #7074), and anti-goat IgG (1:4,000, Sigma-Aldrich, #A5420).
Quantitative RT-PCR
Trizol reagent (Invitrogen) was used to isolate total cellular RNA from cell pellets. After digestion of genomic DNA using Recombinant DNase I (Takara Bio Inc.), a PrimeScript RT reagent Kit (Takara Bio Inc.) was used for reverse transcription into cDNA. Quantitative real-time PCR was performed with a Rotor-Gene Q (QIAGEN) using Luna Universal qPCR Master Mix (New England Biolabs). qPCR primers used in this study were as follows. human CCN1 forward 5ʹ-CCTTGTGGACAGCCAGTGTA-3ʹ and reverse 5ʹ-ACTTGGGCCGGTATTTCTTC-3ʹ; human YAP forward 5ʹ-TAGCCCTGCGTAGCCAGTTA-3ʹ and reverse 5ʹ-TCATGCTTAGTCCACTGTCTGT-3ʹ; human TAZ forward 5ʹ-TCCCAGCCAAATCTCGTGATG-3ʹ and reverse 5ʹ-AGCGCATTGGGCATACTCAT-3ʹ; 18S RNA forward 5ʹ-GTAACCCGTTGAACCCCATT-3ʹ and reverse 5ʹ-CCATCCAATCGGTAGTAGCG-3ʹ.
Statistical analysis
GraphPad Prism 8 software was used to conduct statistical analysis of all data. Data are represented as the mean and SEM. Kaplan–Meier survival curves were generated using GraphPad Prism 8 software and the log-rank test was performed to assess statistical significances between groups. The Student’s t-test was used for comparisons between two groups. Comparisons between multiple groups were performed with one-way ANOVA with Tukey’s multiple comparisons test. Pearson’s correlation test was used to measure the strength of the association between two variables. The chi-squared test was performed to examine differences between categorical variables. P values were designated as *P < 0.05, **P < 0.01 ***P < 0.001, ****P < 0.0001, and ns non-significant (P > 0.05).
Discussion
In this study, we extracted DGC signature genes from transcriptomic profiles of matched pairs of in vitro GSCs and DGCs models. We evaluated the DGC signature genes in single cell RNA-sequencing data, which confirming the presence of cell subpopulations emulated by in vitro culture models within a primary tumor. We found that the DGC gene signature was correlated to macrophage-related genes, the mesenchymal subtype signature, and poor survival. DGCs exhibited significant enrichment of YAP/TAZ/TEAD activity compared with GSCs. We elucidated that DGCs promote macrophage recruitment into GBM tissue through secretion of CCN1.
The significance of GSCs is undeniable considering their capacity for self-renewal, differentiation, and tumor propagation in vivo, and their contribution to therapeutic resistance, immune escape, and angiogenesis [
1,
23]. The focus on GSCs has overlooked the importance of differentiated progeny such as DGCs. A recent study has shown that DGCs also contribute to tumor progression in concert with GSCs through a paracrine loop [
57]. However, the roles of DGCs in the tumor microenvironment of GBM remain obscure. In the present study, we focused on the significant correlation between DGC and macrophage signature genes, and established that DGCs secrete CCN1 to promote macrophage infiltration into the GBM tumor microenvironment. In addition to CCN1, CLCF1, a secreted protein included among the DGC signature genes, is implicated in macrophage activation [
37]. Furthermore, a recent study of medulloblastoma models has also shown that tumor-derived astrocytes, differentiated progeny from tumor progenitors, induce polarization of resident brain microglia towards protumorigenic macrophages by secreting IL-4 [
59]. These findings suggest that not only tumor progenitors such as GSCs, but also differentiated progeny such as DGCs play an essential role in shaping the complex tumor immune microenvironment by promoting macrophage infiltration.
The mesenchymal signature of GBM can be shaped by several factors such as stromal cells, accumulated mutations in tumor cells, the cell of origin, anatomical location/tumor microenvironments, and therapy-induced mesenchymal transition [
4]. The presence of macrophages/microglia is associated with the mesenchymal subtype of GBM [
6,
34,
56]. Genetic deficiency of NF1 attracts macrophages/microglia into tumors and a macrophage/microglia-rich microenvironment also induces a mesenchymal tumor cell phenotype [
56]. We found that DGCs themselves had a mesenchymal gene signature and the DGC signature was anatomically enriched in perinecrotic/hypoxic (pseudopalisading) regions and microvascular proliferative regions. These findings are consistent with prior reports indicating that the mesenchymal signature is enriched in perinecrotic/hypoxic regions and microvascular proliferative regions [
41]. Thus, DGCs may induce a mesenchymal phenotype by attracting macrophages.
DGCs do not form tumors when implanted alone [
51,
57]. This suggest that retrograde dedifferentiation from DGCs to GSCs essentially cannot occur in a tumor [
51]. DGCs successfully implant in brains of recipient mice only when cotransplanted with GSCs, which contribute to GSC-dependent tumor progression [
57]. We also obtained the same result in the present study. Considering our results that DGCs had a mesenchymal gene signature and GSCs had a proneural gene signature when compared each other, increased intratumoral heterogeneity at the earliest time of tumor initiation by cotransplantation of DGCs and GSCs may accelerate tumor progression. This may be consistent with the result of a previous study in which proneural GBM patients with a high proportion of alternate subtype tumor cells had significantly worse outcomes compared with the pure proneural subtype [
38]. Combination therapies that simultaneously target both DGCs and GSCs may be needed to overcome this intratumoral heterogeneity of GBMs.
In GBMs, TAZ is a transcriptional coactivator that drives the gene expression program of mesenchymal differentiation in a TEAD-dependent fashion [
5]. We found that DGCs exhibited significant enrichment of YAP/TAZ/TEAD activity compared with GSCs. Previous studies have reported that serum stimulation in vitro activates AP-1, a transcriptional partner of TEADs, and YAP/TAZ [
61,
62,
64]. Additionally, YAP/TAZ are implicated in mechanotransduction. When cells are cultured on a stiff substrate, YAP/TAZ localize in the nucleus and become transcriptionally active [
17]. Our results are reasonable considering the cell culture conditions under which GSCs grew as spheres in serum-free conditions and DGCs expanded as adherent monolayers under serum-containing conditions. Elevated tissue tension induces a mesenchymal-like phenotype in GBM [
3]. Thus, elevated stiffness may promote YAP/TAZ/TEAD activation of DGCs in GBM tissue.
CCN1, a transcriptional target of YAP/TAZ/TEAD, is secreted from various cell type, including, tumor cells, endothelial cells, fibroblasts, and smooth muscle cells [
19,
36,
40]. CCN1 has been implicated in various cellular processes including leukocyte infiltration, inflammatory process, angiogenesis, and adhesion [
19,
25,
30,
36,
53]. Moreover, we and others have revealed an elevation of CCN1 expression and its correlation with a poor prognosis in various tumors including GBM [
9,
27,
35,
36]. However, which subpopulations of heterogeneous GBM cells secrete CCN1 and its detailed role in the heterogeneous GBM microenvironment has not been fully elucidated. We found that CCN1 was secreted abundantly from a population of DGCs, but not GSCs, which played critical roles in shaping the mesenchymal phenotype through macrophage infiltration into GBM tissue.
The gene signature of in vitro culture models should be interpreted with caution because in vitro DGC and GSC models do not fully recapitulate the heterogeneity of tumor cells within primary GBMs. However, a prior study applied a stemness signature of in vitro culture models to GBM single cell transcriptional profiles and revealed the existence of cell subpopulations emulated by in vitro culture models within a primary tumor [
38]. Interestingly, each in vitro model represents phenotypic extremes of the stemness gradients within GBM tumors [
38]. Using other deposited GBM single cell RNA-sequencing data, we also found clear gradients of DGC and GSC signatures and negative correlations between these signatures in each sample. These results reflect the validity of our approach using the DGC gene signature of in vitro culture models to investigate the biological roles of DGCs in GBMs.
Stem cell-like tumor-initiating cells expressing markers of proneural and mesenchymal transcriptomic subtypes (proneural and mesenchymal GSCs, respectively) have been reported in GBM [
29]. In the present study, we used the proneural subtype of GSC models [
51]. The biological phenotype and anatomical distribution of proneural and mesenchymal GSCs are different [
29]. Accordingly, in the future, studies that focus on the differences among proneural GSCs, mesenchymal GSCs, and DGCs from each subtype of GSCs are needed.
A limitation of the present study is that immune responses, including microglia/macrophages, are different in immunocompetent and immunodeficient mice. BALB/c-nu/nu mice have impaired T-cell functions and an active macrophage system [
12]. Therefore, in the future, we should perform in vivo experiments using immunocompetent syngeneic mouse models.
In conclusion, our results reveal that DGCs contribute GSC-dependent tumor progression by shaping a mesenchymal microenvironment via macrophage infiltration and provide a new insight into the complex GBM microenvironment consisting of heterogeneous mixtures of cells.
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